1. Field of the Invention
The invention relates to a method and a device for calibrating vectorial network analyzers for use with electrical components with differential connections.
2. Discussion of the Background
In precision electronics, vectorial network analyzers (VNA) are used from low frequencies through to high-frequency technology in the GHz range for precision testing of electronic components and components in active and passive circuits and component groups.
A VNA records the so-called scattering parameters of n-port networks (n=1, 2, . . . ), which are optionally converted into 2n-pole parameters (for example, Z-parameters or Y-parameters). However, in the case of mid and high frequencies (fast circuits), these recorded data provide very considerable measurement errors. A so-called system-error correction of the VNA is required to ensure that accurate measurements of fast electronic components can be implemented at all. The measurement accuracy of VNAs depends primarily on the availability of a method for system-error correction. In the context of system-error correction, devices under test, which are known either partially or in their entirety, are tested with regard to reflection and/or transmission behavior within a so-called calibration process. This is known, for example, from DE 199 18 960 A1. Correction data (so-called error parameters or coefficients) are obtained from these measured values via special computational methods. Using these correction data and a corresponding correction calculation, measured values, from which system errors of the VNA and supply lines (couplings=crosstalk, error matchings−reflections) have been removed, can be obtained for any required device under test.
The electrical performance of components and circuits in high-frequency technology is conventionally described via the scattering parameters (also referred to as S-parameters). Rather than linking currents and voltages, the scattering parameters link wave values with one another. This form of presentation is particularly well-suited to the physical conditions of high-frequency technology. If required, the scattering parameters can be converted into other electrical-network parameters, which link currents and voltages.
FIG. 1 shows a two-port, which is to be characterized by its scattering matrix [S]. Let the waves a1 and a2 be the waves travelling towards the two-port; and the waves b1 and b2 be the waves accordingly propagated in the opposite direction. The following relationship applies:
      (                                        b            1                                                            b            2                                )    =            (                                                  S              11                                                          S              12                                                                          S              21                                                          S              22                                          )        ⁢          (                                                  a              1                                                                          a              2                                          )      
One known calibration method for a multi-port model, which is based on the so-called 7-term method, illustrates how the scattering parameters of this so-called 1-mode system can be detected with high precision with network analyzers, which provide a transmitting oscillator.
With passive devices under test, these scattering parameters can be converted into the scattering parameters for components with differential (symmetrical) ports. A full description of these conversions is provided in Heuermann, H., High Frequency Technology, Linear components of highly-integrated high-frequency circuits, Vieweg-Verlag, 2005, ISBN 3-528-03980-9, Chapter 5. This text shows that, alongside the classical ports (also referred to as un-symmetrical, mono-mode ports or single ports), as illustrated in FIG. 1, there are also so-called port pairs, which contain a common-mode port and a differential-mode port. These port pairs are often also referred to as differential or symmetrical ports.
On one hand, components, which provide exclusively port pairs, are introduced in this context by presenting the so-called M-parameters. Only the common-mode and differential modes (2-mode system) occur with these components. Accordingly, the M-matrix for a two-port pair is as follows:
      (                                                      b              1              -                                                                          b              2              -                                                                                      b              1              +                                                                          b              2              +                                            )    =            [                                                                                                        M                    11                    -                                                                                                                    M                    21                    -                                                                                                                                                                M                    12                    -                                                                                                                    M                    22                    -                                                                                                                                                                M                    11                                          -                      +                                                                                                                                        M                    21                                          -                      +                                                                                                                                                                                    M                    12                                          -                      +                                                                                                                                        M                    22                                          -                      +                                                                                                                                                                                                    M                    11                                          +                      -                                                                                                                                        M                    21                                          +                      -                                                                                                                                                                                    M                    12                                          +                      -                                                                                                                                        M                    22                                          +                      -                                                                                                                                                                                    M                    11                    +                                                                                                                    M                    21                    +                                                                                                                                                                M                    12                    +                                                                                                                    M                    22                    +                                                                                          ]        ⁢          (                                                                  a                1                -                                                                                        a                2                -                                                                                                        a                1                +                                                                                        a                2                +                                                        )      a+, b+: common-mode waves, a−, b−: differential-mode waves,
On the other hand, components, in which another un-symmetrical mode (3-mode system) occurs in addition to the common-mode and differential modes, are also introduced with the M-parameters. It is shown in the above context how the scattering parameters of a multi-port measurement can be converted into M-parameters.
These results presented in the above context might suggest that 2-mode and 3-mode systems can be characterised in full in electrical terms, provided they are measured with a multi-port network analyzer, of which the measured values are corrected in the mono-mode system according to a method as disclosed in DE 199 18 960 A1. This assumption is also correct insofar as the device under test is a passive device under test. Using a network analyzer, which provides only one signal source, only a passive device under test can be measured without its properties changing.
In the case of an active component, such as an amplifier, with differential connections, the operating point changes dramatically, if the amplifier is driven in an unsymmetrical manner. Accordingly, with this procedure, a differential amplifier provides different M-parameters, which change significantly, especially in the case of a high-level adjustment.
Methods for calibrating network analyzers with two and more transmitting oscillators, especially those which support a differential excitation for the measurement, are so far not conventionally available.
At present, active components with differential connections are measured using baluns. At low frequencies, broadband transformers are used as baluns, and at high frequencies narrow-band baluns are used. The baluns are connected to every port pair. The device under test can therefore be measured at the correct operating point. However, a large number of measurement errors occur with this procedure. For example, only the differential parameters of the device under test are indicated. These parameters are measured for a fixed-impedance termination of the common mode. In the case of the transformer, this is, for example, an open circuit and does not generally correspond to the values, at which the circuit component is to be considered in the circuit as a whole.
Furthermore, every balancing error of the balun appears as a measurement error. The baluns must be very well matched, which is often not the case in practice. In this context, measurement errors are added, which are difficult to calculate. This procedure therefore corresponds de facto to a scalar measurement, such as those used into the 1970s for purely unsymmetrical devices under test.